Raman spectroscopy is used for a variety of applications, most commonly to study vibrational quanta, such as vibrations in molecules or phonons in solids, although other quantised entities can also be studied. Raman spectroscopy can provide detailed information relating to the physical state of sample materials and can be used to distinguish various states of otherwise chemically identical molecules, such as various molecular isomers, from one another.
Raman spectroscopy finds wide-ranging use in numerous different industries. By way of example, Raman spectroscopy finds application in the pharmaceutical, chemical, bio-analysis, medical, materials science, art restoration, polymer, semiconductor, gemmology, forensic, research, military, sensing and environmental monitoring fields.
Although Raman spectroscopy is an extremely useful analytical tool, it does suffer from a number of disadvantages. The principal drawbacks associated with Raman spectroscopy arise because of the small scattering cross-section. Typically, only 10−7 of the photons incident on the sample material will undergo Raman scattering. Hence, in order to detect Raman scattered photons, Raman spectrometers typically employ high power laser sources and high sensitivity detectors. Not only is the scattering cross-section small in an absolute sense, but it is small relative to Rayleigh scattering in which the scattered photon is of the same energy as the incident photon. This means that there are often problems related to separating out the small Raman signal from the large Rayleigh signal and the incident signal, especially when the Raman signal is close in energy to the incident signal.
High power sources are not only both bulky and expensive, but at very high power the intensity of the optical radiation itself can destroy the sample material, thus placing an upper limit on the optical radiation source intensity. Similarly, high sensitivity detectors are often bulky and expensive, and even more so where forced cooling, such as with liquid nitrogen, is necessary. Additionally, detection is often a slow process as long integration periods are required to obtain a Raman spectrum signal having an acceptable signal-to-noise ratio (SNR).
The problems associated with Raman spectrometry have been known long since C. V. Raman discovered the effect itself in 1928. Since that date, various techniques have been applied to improve the operation of Raman spectrometers.
Certain of the techniques involve the use of metal surfaces to induce surface plasmon resonance (SPR) for more efficient coupling of energy into the sample material. One refinement of this technique involves placing sample material on or near a roughened surface. Such a surface can be formed by the deposition of metallic/dielectric particles, sometimes deposited in clusters. The roughened surface is found to give rise to an enhanced Raman signal, and the technique of using the roughened surface to obtain a Raman spectrum is known as surface enhanced Raman spectroscopy (SERS).
However, while SERS devices can lead to an improved SNR when compared to previous conventional Raman spectrometers, they still suffer from other drawbacks. For example, SERS devices are still not efficient enough to provide a Raman signal without fairly long detector integration times, and still require the use of bulky and expensive detectors. Even at present, an acquisition time for a Raman spectrum of some five seconds is considered to be extremely good. A major problem of many of the presently used SERS systems is the reproducibility of SERS enhancements with repetitions of the same experiment and between different samples. This is largely due to the random distribution and number of hot spots available on the roughened surface and the inability to predetermine their location.
US 2003/042487 describes a method of providing metal objects selected from nanowires, nanorods, and spheroids. This is proposed to surface enhance the spectroscopy of biomolecules in close proximity to the metal objects.
US 2003/059820 discloses the method, system, and method of fabrication of a substrate comprising of metal-coated nanoparticles including nanospheres, nanoparticles, nanorods (see the example shown in FIG. 7(b)).
US 2003/174384 and U.S. Pat. No. 6,699,724 discuss the use of metal-coated spheres and nanoparticles as substrates to enhance the Raman signal from an analyte in close proximity to the substrate.
US 2002/068018 discloses a sensor for the detection of chemical and/or biological compounds consisting of a plurality of high-Q electromagnetic microcavities. It is claimed that a similar effect to SERS results in the detection of the biological agent.
US 2004/180379 discloses a nanobiosensor based on SERS, while U.S. Pat. No. 6,579,721 discusses a method of biosensing using surface plasmon resonances from perforated metal film located on a glass substrate (as illustrated in FIG. 7(a)). U.S. Pat. No. 6,759,235 discloses a device consisting of closely packed array of collimated holes distributed across a surface used for immobilizing beads to generate spectra in response to some external excitation energy.
Finally, M. Kahl et al. (Phys. Rev. B, vol. 61, no. 20, art. 14078) discuss the use of metal coated 1 D gratings to enhance the Raman signal off a surface and P. Etchgoin et al. (J. Chem. Phys., vol. 119, no. 10, p. 5281) elaborate on the electromagnetic contribution to SERS in view of photonic crystal concepts through an analysis of the energy distribution and spatial localization of surface plasmon resonances to provide a qualitative understanding of some known SERS phenomena. The success of nonresonant excitation in the electromagnetic contribution to SERS is explained and physical phenomena utilizing the stimulation or inhibition of the Stokes/anti-Stokes fields are proposed. W. L. Barnes et al. (Nature, vol. 424, p. 824) provide an overview on the impact of surface plasmons with regard to various applications.
As such, the role of surface plasmon polaritons (SPPs) in the understanding of extraordinary transmission properties of light through a metal film perforated with subwavelength apertures has been recognized, and the application of this effect in the context of SERS has been identified. However, the combination of SPPs propagating inside holes, SPPs propagating on metal interfaces and localized SPs bound to surface defects (and couplings thereof) have not been explored in the context of enhancing the Raman signal for use as a biosensor. Additionally, the potential benefits of separating the light collection and extraction from the Raman signal generation, and the possible monolithic integration of signal preparation functions before and after the Raman generating process, have not been clearly identified by researchers in the field.